尝试在Kivy Python中混合使用box布局和floatlayout，但是当我运行时，我得到一个空白的黑框

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.button import Button

class BoxLayoutExample(BoxLayout):
    pass

class FloatLayoutExample(FloatLayout):
    pass

class BoxLayoutExample(BoxLayout):
    def __init__(self, **kwargs):
        super(BoxLayoutExample, self).__init__(**kwargs)
        self.orientation = 'vertical'
        
        button1 = Button(text='Button 1')
        button2 = Button(text='Button 2')
        
        self.add_widget(button1)
        self.add_widget(button2)

class FloatLayoutExample(FloatLayout):
    def __init__(self, **kwargs):
        super(FloatLayoutExample, self).__init__(**kwargs)
        
        button1 = Button(text='Button 1', pos=(100, 100))
        button2 = Button(text='Button 2', pos=(200, 200))
        
        self.add_widget(button1)
        self.add_widget(button2)

class MyApp(App):
    def build(self):
        box_layout = BoxLayoutExample()
        float_layout = FloatLayoutExample()
        
        return box_layout

if __name__ == '__main__':
    MyApp().run()